Almost all the microwave filters and duplexers for high power transmitting applications require integration of directional couplers. Though the couplers are probably the simplest in terms of complexity in a filter integrated system, coupler failures have become one of the major bottlenecks in production. Coupler failures for directivity and match are the two major causes of yield problems. Though the couplers may look completely different after they are implemented to fit into a certain geometry, many of them fall in one of the following the categories: slabline, stripline, micro-stripline, coupled line, multi-hole wave guide couplers, rat race, the barrel coupler etc. Each has its own advantages, disadvantages and idiosyncrasies. A theoretical discussion of each of these coupler types can be found in many text books and papers, J. A. G. Malherbe, “Microwave Transmission Line Couplers”, Artech House, 1988; D. M. Pozar, “Microwave Engineering”, Addison-Wesley Publishing Company, 1993; and G. L. Matthaei, L. Young and E. M. T. Johes, Microwave Filters, Impedance-Matching Networks and Coupling Structures, Artech House Dedham, Mass. 1964. A good discussion of some of the coupler types can be found in Joe Nicewicz, “Directional Coupler Techniques”, Filtronic Engineering Conference 1999 and Puma Subedi, “The Barrel Coupler”, Filtronic Engineering Conference 2000.
A coupler referred to as a “P coupler” has been provided which has several advantages. The “P” coupler provides a good broadband match and the same coupler can be tuned for any band within a several GHz window. Not only the performance of the “P” coupler exceeds the performance of the Barrel Coupler (Puma Subedi, “The Barrel Coupler”, Filtronic Engineering Conference 2000), the cost estimate is lower than half of the Barrel coupler. The directivity and the couplings are tunable parameters and usually 30-dB of directivity with at least 20-dB of match is easily achieved. These couplers can be tuned from upper around 10 dB to 50-dB values. The same barrel coupler that works well in all the Cellular, DCS, PCS and the UMTS bands has been developed and the results and design details are discussed in Puma Subedi, “The P-Coupler”, 2002. Additionally, the P-Coupler can be used in extremely high peak power conditions unlike the Barrel Coupler where the Barrel housing can come too close to the main transmission line. In utilizing this coupler, the loss of the thru line is only the loss of an air filled coaxial line plus whatever the coupling loss is since this is an airline coupler. This coupler uses only 0.75-inches (19.05 mm) diameter of x, y real-state and the coupling direction is reversible.
Despite the advantages of the known “P” coupler design, the present applicant has discovered that non-obvious improvements can be made which have advantages for cost, manufacturability and allow smaller size and weight.